Process for preparing partially hydrogenated butadiene polymers

ABSTRACT

The present invention relates to a process for preparing a partially hydrogenated butadiene polymer comprising no more than 3 mol % 1,2-butadiene recurring units (A) and no more than 3 mol % ethylene recurring units (D), calculated on the total content mol/mol of recurring units, wherein a butadiene polymer comprising 1,2-butadiene recurring units (A) and 1,4-butadiene recurring units (B) is hydrogenated in the presence of hydrogen and a titanium, zirconium- and/or hafnium-based metallocene compound as hydrogenation catalyst and a cocatalyst, characterized in that: a) the hydrogenation catalyst has a reaction rate ratio of r1/r2 greater than 5, wherein r1 and r2 are the hydrogenation rates of recurring units (A) and (B) respectively at the same reaction conditions, b) the cocatalyst is an alkali metal hydride, added as such of prepared in situ, and c) hydrogenation of the conjugated diene is carried out until at least 97 % of recurring units (A) have been hydrogenated.

FIELD OF THE INVENTION

[0001] The present invention relates to a process for preparingpartially hydrogenated butadiene polymers. In particular, the presentinvention relates to a process for preparing a partially hydrogenatedbutadiene polymer comprising no more than 3 mol % 1,2-butadienerecurring units (A) and no more than 3 mol % ethylene recurring units(D), calculated on the total content mol/mol of recurring units. More inparticular, the present invention relates to a process for preparingblock copolymers comprising at least one partially hydrogenatedpolybutadiene block comprising no more than 3 mol % 1,2-diene recurringunits (A) and no more than 3 mol % ethylene recurring units (D),calculated on the total content mol/mol of recurring units.

BACKGROUND OF THE INVENTION

[0002] In is known that conjugated diene polymers comprise a mixture ofrecurring units of structures which in the art are referred to as (cisor trans) 1,4- and 1,2-structures. The former results in unsaturation inthe polymer backbone, the latter results in the attachment ofunsaturated groups (vinyl groups) to the backbone. Conjugated dienepolymers with selectively hydrogenated 1,2-butadiene (butylene) unitshave been found to have a number of desirable properties. For instance,the absence of vinyl groups results in improved heat resistance, whilethe remaining essentially unaffected unsaturation in the backboneresults in good processability and good elastomeric properties at lowtemperature.

[0003] In U.S. Pat. No. 3,700,748 block polymers are provided havingsubstantially improved capability of forming stable elastomeric polarderivatives, these block polymers being prepared by block polymerizing amonovinyl arene with butadiene, the butadiene block containing betweenabout 8 and 80 percent 1,2-structure and thereafter selectivelyhydrogenating (in the presence of a nickel based catalyst) so as tosubstantially eliminate the unsaturation in the pendant vinyl groups.This process, however, is not very selective since hydrogenatingbutadiene blocks having 1,2-structure, as well as those having a1,4-structure.

[0004] U.S. Pat. No. 3,663,635, DE 3401983, U.S. Pat. No. 5,039,755,U.S. Pat. No. 513272, EP 339986, EP 434469, EP 544304, EP 795564, EP810231, and WO 9525130 described catalyst systems that are suitable forthe hydrogenation of conjugated diene (co)polymers. These catalystsystems are prepared by reacting a titanocene or similar group 4metallocene (i.e., a ferrocene-like molecule based on a group 4 metaland 2 η⁵ ligands), with a metal hydride or an organo-metallic compoundand contacting (activating) the resulting catalyst mixture withhydrogen. These catalyst systems have a very high hydrogenationactivity, but none are (known to be) capable of selectivelyhydrogenating said conjugated diene (co)polymers into partiallyhydrogenated (co) polymers.

[0005] In a paper, entitled “Metallocenes: homogeneous catalysts forelastomer hydrogenation”, the authors M. D. Parellada, J. A. Barrio, J.A. Delgado (Rev. R. Aced. Cienc. Exactas, Fis. Nat. Madrid (1993),87(1), 127-9) describe the advantages of a metallocene type catalyst forhydrogenating styrenic block copolymers over Ziegler type hydrogenationcatalysts. It is said that replacement of a cyclopentadienyl (Cp) ringby a pentamethylcyclopentadienyl (Cp*) ring in these titanium complexesbrings about greater stability and more selectivity, first hydrogenatingvinyl bonds rather than the olefin bonds within the copolymers. Asuitable process for the preparation of partially hydrogenated polymersis, however, not disclosed.

[0006] EP 545844 describes a process for partial hydrogenation of astyrenic block copolymer in the presence of Cp*CpTiX2 and n-butyllithium. Although all or nearly all of the 1,2-polybutadiene ishydrogenated, so is also a major portion of the 1,4-polybutadiene (cf.Examples 12 to 15). This application rather illustrates the difficultyof obtaining (highly selective) partially hydrogenated polymers whereinthe 1,2-butadiene recurring units are, but the 1,4-butadiene recurringunits are not hydrogenated.

[0007] EP 584860 describes a process for partial hydrogenation ofconjugated diene polymers, that selectively hydrogenates butadieneunits, but not (or less so) isoprene units. This process too is not veryselective as regards hydrogenating butadiene blocks having1,2-structure, respectively 3,4-structure (in case of isoprene) only.

[0008] In JP 04096904 Asahi describes a method for hydrogenating olefincompounds, using a titanium-, zirconium- or hafnium-based metallocenecompound as hydrogenation catalyst and in the presence of a reducingcompound. For instance, this application describes the hydrogenation of7 different (block) copolymers with Cp*₂TiCl₂ and dibutyl magnesium orn-butyl lithium as catalyst. Hydrogenation in each instance was nearlycomplete; preparation of the aforementioned partially hydrogenatedconjugated diene polymers is not described.

[0009] In EP 302505 Asahi describes partially hydrogenated butadienepolymers or partially hydrogenated random butadiene/styrene copolymersand a process for preparing the same. However, although “any catalystsand any production methods may be utilized” (page 5, line 8), thespecification also indicates that “the selective partial hydrogenationof the present invention must be practiced under extremely mild reactionconditions . . . ” (page 5, lines 12-13). It would therefore appear thatthe preparation of partially hydrogenated conjugated diene polymersrequires rigid control of the reaction conditions. Besides, thehydrogenation illustrated in this patent is not selective to the1,2-conjugated diene recurring unit: without even being close tocomplete hydrogenation of the “vinyl-linkage moiety [B]”, hydrogenationof the “whole butadiene portion [A]” is already greater than the“vinyl-linkage content [V]” (cf. Table 1 of this patent), whereas if the“vinyl-linkage moiety [B]” is completely hydrogenated, than also the1,4-linkage content is substantially reduced (cf. Tables 5 to 8 of thispatent).

[0010] In JP 08106490 Asahi describes an improvement on said process,thus producing a partially hydrogenated butadiene polymer with selectivehydrogenation of the 1,2-diene recurring units. According to thisapplication, at least 90% of the 1,2-diene recurring units must behydrogenated, with a catalyst having an r1/r2 ratio greater than 5,wherein ‘r1’ corresponds with the reaction rate for the hydrogenation ofthe 1,2-diene recurring units and wherein ‘r2’, at identical reactionconditions, corresponds with the reaction rate for the hydrogenation ofthe 1,4-diene recurring units. A compound comprising titanium ligated bya substituted metallocene is provided as an example of a suitablehydrogenation catalyst. However, stringent control of the hydrogenationreaction still appears to be necessary, to avoid the hydrogenation ofthe 1,4-diene recurring units.

[0011] In U.S. Pat. No. 5,925,717 the hydrogenation of hydrogenterminated SBS block copolymers is disclosed, in the presence ofoptionally substituted titanium indenyl and/or bis indenyl compounds ascatalyst. Such catalysts are said to be equally active to 1,4-conjugateddiene recurring units as 1,2-conjugated diene recurring units (column 6,lines 52-58).

[0012] It is therefore an object of the present invention to provide anattractive process, that does not require rigid control of the reactionconditions, to prepare the partially hydrogenated conjugated dienepolymers described herein before.

BRIEF SUMMARY OF THE INVENTION

[0013] Accordingly, the invention provides a process for preparing apartially hydrogenated butadiene polymer comprising no more than 3 mol %1,2-butadiene recurring units (A) and no more than 3 mol % ethylenerecurring units (D), calculated on the total content mol/mol ofrecurring units, wherein a butadiene polymer comprising 1,2-butadienerecurring units (A) and 1,4-butadiene recurring units (B) ishydrogenated in the presence of hydrogen and a titanium-, zirconium-and/or hafnium-based metallocene compound as hydrogenation catalyst anda cocatalyst, characterized in that:

[0014] a) the hydrogenation catalyst has a reaction rate ratio of r1/r2greater than 5, preferably greater than 15, wherein r1 and r2 are thehydrogenation rates of recurring units (A) and (B) respectively at thesame reaction conditions,

[0015] b) the cocatalyst is an alkali metal hydride, added as such orprepared in situ, and

[0016] c) hydrogenation of the conjugated diene is carried out until atleast 97% of recurring units (A) have been hydrogenated.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The group 4 metallocene may be similar to the metallocenesmentioned in documents mentioned above. Such compounds may berepresented by the following general formula:

L ₂ M X ₂  (II)

[0018] wherein M represents a metal chosen from Ti, Zr and Hf; each Xindependently is selected from hydrogen, C₁-C₁₂ hydrocarbon groups,C₁-C₁₂ hydrocarbonoxy groups, C₁-C₁₂ hydrocarbonylsilyl groups, halogengroups and carbonyl groups; and each L independently is selected from acyclopentadienyl group, an indenyl group or a 5-membered heterocyclicgroup, each bearing at least one substituent, wherein the substituent isselected or the substituents are each independently selected from C₁-C₁₂hydrocarbon groups, C₁-C₁₂ hydrocarbonoxy groups, halogen groups andcarbonyl groups, and bulky substituents containing one or moreheteroatoms such as C₁-C₁₂ hydrocarbonylsilyl groups, a C₄-C₁₀dihydrocarbylamino group.

[0019] Each ligand L may be part of a fused ring system, i.e., anindenyl or fluorenyl group, as disclosed in EP 795564, EP 830895 andU.S. Pat. No. 3,663,635 herewith incorporated by reference.

[0020] Each ligand L may (also) be a heterocyclic ring system, i.e., aphospholyl group, as disclosed in EP 810213 herewith incorporated byreference.

[0021] Both ligands L may (also) be linked together, i.e., by adialkylsilyl group , as disclosed in EP 545844, herewith incorporated byreference.

[0022] Suitable substituents whereby groups L are substituted, includehalogen atoms, hydrocarbyl groups optionally containing heteroatoms ofup to 12 carbon atoms. Such heteroatoms include nitrogen, oxygen,phosphorus, silicon and the halogens.

[0023] Preferred substituents are C₁-C₆ alkyl groups, C₁-C₆ alkoxygroups, C₆-C₁₀ aryl groups, C₆-C₁₀ aryloxy groups, bis(C₁-C₄ alkyl)silylgroups and tris(C₁-C₄ alkyl)silyl groups and chlorine atoms.

[0024] The number of substituents may be easily determined by empiricalmethods. For instance, in case of a metallocene comprising twosubstituted indenyl or fluorenyl groups a single substituent (at the 1-,2-, or 3- position) will normally suffice to cause a r1/r2 ratio that isgreater than 5. Suitable examples of mono-substituted indenyls include:methyl-; isopropyl-; tert-butyl-; phenyl-; chloro-; bismethylsilyl;bisethylamino-; pentaflurorphenyl-; and bis(n-butyl)phosphinoindenyl(either in the 1- or 2- position), and the like. Suitable examples ofthe disubstituted indenyls include 1,2-dimethyl-; 1,3-dimethyl-;1-methyl,2-ethyl-; and 1-methyl,2-chloroindenyl and similar compounds.Indeed, a titanium metallocene comprising two 1-trimethylsilyl-indenylgroups has a r1/r2 ratio of greater than 15, i.e. about 22 (determinedat ¹H NMR analysis of reaction products made at 70° C. and 30 kg/cm²hydrogen), which is in the preferred range. In case of a substitutedphosphoryl group, generally at least two substituents will be required.Finally, in case of a substituted cyclopentadienyl group, generally atleast 3 substituents will be required, preferably at the 1-, 2-, and4-positions. Examples of suitable substituted cyclopentadienyl groupsinclude 1,2,4-trimethyl-; 1,2,4-tris(tert-butyl)-; tetramethyl-; andpentamethylcyclopentadienyl. Use of pentamethylcyclopentadienyl asligand will result in a metallocene having a r1/r2 ratio of about 230.If the substltuents themselves are very bulky, then even lesssubstituents may be sufficient.

[0025] Examples of preferred ligands hence include:pentamethylcyclopentadienyl, 1-trimethylsilylindenyl, 2-methylindenyl;1,2-dimethylindenyl; 2-phenylindenyl; 1,3-dimethylindenyl;1,3-di(tert-butyl)indenyl; 2-tert-butylindenyl; and1,2,3-trimethylindenyl and the like.

[0026] The expressions hydrocarbyl and hydrocarboxyl used above includealkyl and alkoxy groups (cyclic, linear or branched); aryl and aryloxygroups and substituted variants thereof.

[0027] Preferably each X is a halogen atom, more preferably a chlorideatom. Therefore, the most preferred group 4 metallocene is bis(pentamethylcyclopentadienyl) titanium dichloride (Cp*₂TiCl₂).

[0028] The metallocene is used in amounts of from 0.001 to 10, typicallyfrom 0.005 to 50 mmol per 100 g of unsaturated compound, and preferablyin amounts in the range of from 0.01 to 1 mmol/100 g. More may be usedto speed up the hydrogenation process, but also—given the great activityof the catalyst system—less may be used, to reduce the cost andenvironmental impact of the catalyst system.

[0029] The more common alkali metal hydride used as cocatalyst islithium hydride, as lithium compounds are frequently used aspolymerization initiator. The lithium hydride may be added as is, butalso be made in situ by termination of the living polymer (noticeable bythe disappearance of the typical orange color) or by reaction of anadded lithium alkyl with hydrogen. The preparation of the lithiumhydride by termination of the living polymer is preferred. However thecocatalyst may also be a sodium or potassium hydride.

[0030] The initial molar ratio of cocatalyst over the metallocenehydrogenation catalyst may vary widely. Thus, this molar ratio may varyfrom 2 to 100, for instance from 4 to 50.

[0031] It is the merit of the present invention that the hydrogenationreaction is relatively straightforward and uncomplicated. For instance,in the preferred embodiment a cement containing living polymer isterminated by addition of hydrogen and vigorous stirring until thetypical color of living polymer has disappeared. The metallocene iseither dissolved in a suitable, inert solvent to which subsequently thecocatalyst is added or vice versa. No specific pressure and temperatureconditions apply. For instance, the metallocene and cocatalyst may bereacted by contacting these components for about 1 to 60 minutes,preferably for 5 to 20 minutes at a temperature in the range of 20-90°C. and at a pressure in the range of 0.0 to 51 kg/cm².

[0032] The catalysts of this invention can be used to hydrogenate thedouble bonds of butadiene polymers, and copolymers. In particular, theymay be used for the selective hydrogenation of such polymers andcopolymers of medium molecular weight, i.e. having a weight averagemolecular weight in the range from 500 to 1,000,000. Copolymers ofparticular interest are copolymers of 1,3-butadiene and anotherconjugated diene and/or a vinylaromatic compound such as styrene oralpha-methylstyrene and/or an acrylic compound such as alkyl acrylate oralkyl methacrylate. Among these copolymers are included randomcopolymers in which the comonomers are randomly distributed along thepolymer chain, cross-linked copolymers and pure or gradual blockcopolymers.

[0033] The block copolymers are especially interesting since some ofthem are thermoplastic elastomers usable from the industrial point ofview. Such block copolymer consist of:

[0034] (a) at least one polymer block predominantly made of one or morevinylaromatic compounds or predominantly made of one or more acryliccompounds; and

[0035] (b) at least one polymer block predominantly made of butadiene.The expression “predominantly” in this respect means for at least 80 mol%. Among these block copolymers are included linear and branched andradial and star copolymers. Another group of interesting blockcopolymers includes polymers consisting of at least one polymer blockpredominantly made of butadiene, and at least one polymer blockpredominantly made of one or more conjugated dienes other thanbutadiene. The block copolymers used preferably in this invention arethe so-called styrenic block copolymers that contain between 10 to 90wt. % of vinylaromatic compounds. The preferred copolymers are thosewhich contain approximately 25 to 80 percent of 1,2-vinylic bonds in thebutadiene block.

[0036] The butadiene polymers and copolymers that can be hydrogenatedaccording to this invention can be obtained by known polymerizationmethods such as anionic polymerization, cationic polymerization,coordination polymerization, or radical polymerization, etc. The anionicpolymerization is especially interesting in producing polymers andcopolymers that can be hydrogenated according to the invention. Amongthe initiators that can be used, the organolithium compounds arepreferred, particularly a butyllithium compound.

[0037] Hydrogenation is preferably carried out in solution in an inerthydrocarbon, preferably the same hydrocarbon solvent employed duringpolymerization. The term “inert solvent” means an organic solvent thatdoes not react with any of the reactants that participate in thereaction. Examples of these inert solvents that are recommended insidethe frame of this invention are the aliphatic hydrocarbonad andcycloaliphatic hydrocarbons such as n-hexane, n-oxtance, isooctane,cyclohexane, methylcyclopentane, cyclopentance, ethers such astetrahydrofurane, aromatic hydrocarbons such as benzene, toluene, xylenethat are not hydrogenated in the selected reaction conditions, andmixtures of these compounds.

[0038] Conventional hydrogenation conditions may be applied. Forinstance, suitable hydrogen pressures are between 1 and 70 kg/cm²,preferably between 5 and 50 kg/cm². Suitable reaction temperatures varyfrom 20 to 150° C., preferably between 50 and 120° C. Ideally, theconditions are selected such as to approach near full conversion of therecurring units (A) (i.e., better than 97%, preferably better than 98%,more preferably better than 99%) with hardly any conversion of therecurring units (B) (i.e., no more than 3%, preferably no more than 2%,more preferably no more than 1%).

[0039] The product resulting from this selective hydrogenation may beexamined by ¹H NMR to determine the presence or absence of any vinylgroups. The product resulting from this selective hydrogenation may beexamined by ¹H NMR analysis to determine the presence or absence of anyvinyl groups. In the usual ¹H NMR analysis, any 1,4-structure will showa chemical shift between 5.15 and 5.46 ppm. The 1,2-structure shows twoshifts namely between 4.75 and 5.10 ppm and between 5.46 and 5.73 ppm.It is desirable in the final product that substantially no signal isobserved within the latter two chemical shift regions in the ¹H NMRspectrum.

[0040] The hydrogenation products may be easily isolated from thesolvent through known processes such as distillation, precipitation,etc.

[0041] The invention will now be illustrated by means of the followingexamples.

[0042] Polymer Preparation

[0043] A polystyrene-polybutadiene-polystyrene block copolymer (PolymerA) of medium vinyl content and having an apparent molecular weight of100.000 (as measured with gel chromatography (GPC) using polystyrenecalibration standards) was prepared in a stainless steel reactor bysequential anionic polymerization using sec- butyllithium as theinitiator. The polymerization was conducted in cyclohexane, to which hadbeen added 150 ppm of diethoxypropane (DEP). At the end of thepolymerization reaction the reactor was sparged with hydrogen toterminate the living SBS-Li polymer and produce a cement comprising theaforementioned polymer (SBS) and LiH.

[0044] In a similar fashion cements comprising high vinyl polymersPolymer B and D were made (using, however, 300 ppm of DEP). Using theprocess of Polymer A also a cement comprising a higher molecular weightpolymer, Polymer C, was made. Polymer E was made analogous to Polymer A.The results are in Table 1.

EXAMPLE 1

[0045] Selective hydrogenation of medium vinyl SBS block copolymer withbis (pentamethylcyclopentadienyl)titanium dichloride (CP*₂TiCl₂).

[0046] A stainless steel reactor was charged with 800 grams of cement,comprising polymer A. The temperature of the reactor was set to 70° C.and the reactor was pressurized to 10 kg/cm² of hydrogen to saturate thecement.

[0047] Meanwhile a suspension of 45 mg (0.12 mmol) of Cp*₂TiCl₂ in 10 mlof cyclohexane was prepared. The catalyst suspension was added to thereactor and hydrogen pressure was raised to 30 kg/cm². The hydrogenationwas allowed to proceed for 2 hours, during which period samples weredrawn from the reactor and analyzed by ¹H NMR to determine theconversion of the olefinic double bonds. Results are shown in Table 2.

EXAMPLES 2-5

[0048] In a manner similar to Example 1, cements containing Polymers B-Ewere selective hydrogenated. In Examples 2, 3 and 5-9, the temperaturewas set to 90° C., whereas in Example 4, the temperature was set to 120°C. The amount of catalyst varied from 45 mg (0.12 mmol) Cp*₂TiCl₂(Example 2); 52 mg (0.14 mmol) Cp*₂TiCl₂ (Example 3); 60 mg (0.16 mmol)Cp*₂TiCl₂ (Example 4); 31 mg (0.06 mmol)bis(1-trimethylsilylindenyl)TiCl₂ (Example 5); 34 mg (0.09 mmol)bis(2-methylindenyl)TiCl₂ (Example 6); 67 mg (0.17 mmol)bis(1,2-dimethylindenyl)-TiCl₂ (Example 7); ); 66 mg (0.13 mmol)bis(2-phenyl-indenyl)TiCl₂ (Example 8); 67 mg (0.17 mmol)bis(1,3-dimethylindenyl)TiCl₂ (Example 9). In Examples 5, 7 and 8, thereaction was allowed to proceed for 1 hour instead of 2 hours. Theseresults are also included in Table 2.

COMPARATIVE EXAMPLE 1

[0049] In a manner identical to Example 1, but for the use of asuspension of 39 mg (0.11 mmol) of bis(indenyl)-titanium dichloride in10 ml of cyclohexane, Polymer A was selectively hydrogenated. Thismetallocene has a r1/r2 ratio of 2. The hydrogenation was allowed toproceed for 1 hour. The results are shown in Table 3.

COMPARATIVE EXAMPLE 2

[0050] In a similar manner to Example 1, conducted at 90° C. a cementcomprising Polymer D was hydrogenated using a suspension of 40 mg (0.15mmol) of bis(methylcyclopentadienyl)titanium dichloride in 10 ml ofcyclohexane. This metallocene has a r1/r2 ratio of 3. The hydrogenationwas allowed to proceed for 1 hour. The results are shown in Table 3.

Reference Example 1

[0051] In Examples 20 to 22 and Comparative Examples 1 and 2 of JP2-214418 an SBS block copolymer, terminated with methanol, ishydrogenated under conditions similar to Example 1 using Cp*₂Ti(phenyl)₂or Cp*₂Ti(tolyl)₂ either or not in the presence of a cocatalyst. Underall circumstances, whether mild or sever, more than the 1,2-vinyl bondcontent was hydrogenated.

[0052] Conclusion

[0053] The object of the present invention, providing a process thatwill lead to a partially hydrogenated butadiene polymer comprising nomore than 3 mol % 1,2-butadiene recurring units (A) and no more than 3mol % ethylene recurring units (D), calculated on the total contentmol/mol of recurring units, is achieved by the examples according to theinvention. If the reaction rate ratio is not high enough, as incomparative examples 1 and 2, then—despite the presence of lithiumhydride—the selectivity is inadequate. On the other hand, the prior artabundantly illustrates that merely using titanium-based substitutedmetallocene compounds as hydrogenation catalyst neither does the trick.TABLE 1 Polymer preparation Styrene content Vinyl content Solids content[LiH] Polymer (% wt.) (% wt.) (% wt.) (mmol/L) A 30 41 14 1.7 B 29 69 141.7 C 28 8 8 0.8 D 30 65 18.5 2.3 E 30 45 13.2 1.6

[0054] TABLE 2 Example [cat] 1,2-vinyl butylene 1,4-butadienepolyethylene No. Polymer (mmol/100 g) LiH/Ti mol % mol % mol % mol % 1 A0.11 14 0 41 58 1 2 A 0.11 14 0 41 58 1 3 B 0.13 12 0 69 30 1 4 C 0.25 50 8 91 1 5 D 0.04 38 0 65 32 3 6 E 0.10 18 1 44 54 1 7 E 0.18 10 0 45 532 8 E 0.14 12 0 45 51 4 9 E 0.18 10 1 44 54 1

[0055] TABLE 3 Example [cat] 1,2-vinyl butylene 1,4-butadienepolyethylene No. Polymer (mmol/100 g) LiH/Ti mol % mol % mol % mol % C 1A 0.10 15 0 41 19 40 C 2 D 0.10 15 5 60 13 22

1. Process for preparing a partially hydrogenated butadiene polymercomprising no more than 3 mol % 1,2-butadiene recurring units (A) and nomore than 3 mol % ethylene recurring units (D), calculated on the totalcontent mol/mol of recurring units, wherein a butadiene polymercomprising 1,2-butadiene recurring units (A) and 1,4-butadiene recurringunits (B) is hydrogenated in the presence of hydrogen and a titanium-,zirconium- and/or hafnium-based metallocene compound as hydrogenationcatalyst and a cocatalyst, characterized in that: a) the hydrogenationcatalyst has a reaction rate ratio of r1/r2 greater than 5, wherein r1and r2 are the hydrogenation rates of recurring units (A) and (B)respectively at the same reaction conditions, b) the cocatalyst is analkali metal hydride, added as such or prepared in situ, and c)hydrogenation of the conjugated diene is carried out until at least 97%of recurring units (A) have been hydrogenated.
 2. Process as claimed inclaim 1, wherein the cocatalyst (I) is added as is; or (II) is in-situformed by termination with hydrogen of a living butadiene polymer,produced by anionic polymerization in the presence of an alkali metalcompound; or (III) is in situ formed by termination of an added alkalimetal compound.
 3. Process as claimed in any one of claims 1 or 2,wherein the alkali metal hydride is lithium hydride.
 4. Process asclaimed in any one of claims 1 to 3, wherein the initial molar ratio ofcocatalyst over metallocene hydrogenation catalyst varies from 2 to 100.5. Process as claimed in any one of claims 1 to 4, wherein thehydrogenation catalyst may be represented by the general formula: L ₂ MX ₂  (II) wherein M represents a metal chosen from Ti, Zr and Hf; each Xindependently is selected from hydrogen, C₁-C₁₂ hydrocarbon groups,C₁-C₁₂ hydrocarbonoxy groups, C₁-C₁₂ hydrocarbonylsilyl groups, halogengroups and carbonyl groups; and each L independently is selected from acyclopentadienyl group, a indenyl group or a 5-membered heterocyclicgroup, each bearing at least one substituent, wherein the substituent isselected or the substituents are each independently selected fromhydrogen, C₁-C₁₂ hydrocarbon groups, C₁-C₁₂ hydrocarbonoxy groups,halogen groups and carbonyl groups, and bulky substituents containingone or more heteroatoms such as C₁-C₁₂ hydrocarbonylsilyl groups, aC₄-C₁₀ dihydrocarbylamino group.
 6. Process as claimed in claim 5,wherein a hydrogenation catalyst is used wherein M is a titanium atom.7. Process as claimed in claim 5, wherein each group L is selected frompentamethylcyclopentadienyl, 1-trimethylsilylindenyl, 2-methylindenyl;1,2-dimethyl-indenyl; 2-phenylindenyl; 1,3-dimethylindenyl;1,3-di(tert-butyl)indenyl; 2-tert-butylindenyl; and1,2,3-trimethylindenyl.
 8. Process as claimed in any one of claim 1 to7, wherein the hydrogenation is conducted at a hydrogen pressure ofbetween 1 and 70 kg/cm² and a reaction temperature of between 20 and150° C.
 9. Process as claimed in any one of claims 1 to 8, wherein thepartially hydrogenated butadiene polymer is a partially hydrogenatedblock copolymer having at least one partially hydrogenated polybutadieneblock.
 10. Product prepared by a process as claimed in any one of claims1 to 9.